This invention relates to rotary hammers and in particular to hammers that incorporate an air cushion hammering mechanism, and to tool holders for use in such hammers.
Such hammers will normally include a tool holder that can hold a hammer bit or chisel bit for acting on a workpiece, and an air cushion hammering mechanism which comprises a piston, a beat piece and normally also a ram that are slidably located in a spindle so that reciprocation of the piston in the spindle will cause the beat piece to hit the end of the bit located in the tool holder with a considerable force, thereby to cause the bit to impact the workpiece. Such hammers are well known, and are described, for example in EP-A-0 014 760, EP-A-0 429 475 and EP-A-0 759 341. The tool holders and the bits are arranged so that the bit is able to move to a limited extent in the axial direction under the force of the impact from the beat piece, and so chip at the workpiece. A number of designs have been proposed for the bits, and the particular design will depend, among other things, on the size of the hammer.
A tool bit design that has become extremely popular during the last few years is that described in DE-A-25 51 125. This form of bit is known under the designation "SDS Plus" and has a circular shank of approximately 10 mm diameter extending for approximately 50 mm (beyond which the diameter may increase or decrease depending on the design of the bit). The shank of the bit has a pair of driving grooves that extend in the axial direction to the rear end of the bit so that the bit shank can engage driving splines when the bit is inserted in the tool holder in order to enable the bit to be rotated instead of being impacted or in addition to being impacted as desired by the operator. The shank also contains a pair of axially extending retaining grooves, each being approximately 20 mm in length located toward the rear end of the shank (i.e. the end that is inserted in the tool holder), which grooves, or one of which grooves, receives a retaining element of the tool holder that extends radially into the bore of the cylinder. The retaining grooves each have closed ends, that is to say, the grooves do not extend to the rear end of the bit shank, so that the closed ends limit axial movement of the bit in the tool holder by hitting the retaining element(s) as the bit slides along the cylinder to its forward position. It is necessary to be able to retract the retaining elements so that they do not extend radially into the bore of the cylinder when the bit shank is inserted into the cylinder and removed from the cylinder. To this end, the tool holder is typically provided with a sleeve that is located on the outer surface of the tool holder and is axially slidable to a defined extent by the operator in order to present an internal recess to the retaining element(s) into which the retaining element(s) can move during insertion and extraction of the bit. In a typical tool holder design, the retaining element is in the form of one or more balls that extend into the bore of the cylinder by an amount that is less than the radius thereof, so that, once the recess has been presented to the retaining element, the retaining element can be pushed radially into the recess simply by insertion or extraction of the bit shank.
When the hammer is being operated to act on a workpiece, the impact of the beat piece on the bit is largely absorbed by the workpiece. However, when the hammer is removed from the workpiece by the operator, the beat piece will hit the bit one last time and knock the bit into the forwardmost position in the tool holder so that the hammer comes into so-called "idle mode" and the beat piece cannot hit it again until the operator re-engages the bit in the workpiece. When the bit is struck by the beat piece during this last impact, the bit is thrust rapidly forward until the rear end of one or more of the retaining grooves strikes the retaining element which arrests the forward motion of the bit by transferring its momentum to the rest of the tool holder. This is normally achieved by the retaining element striking a restraining element that is fixed in the tool holder.
It is now intended to employ bits of the type that have a shank with one or more retaining grooves, such as SDS Plus, in hammers larger than those in which they were originally employed, which generate larger impacts on the drill or chisel bit inserted in the tool holder, and/or to employ this type of coupling of the bit shank with bits of larger mass than before. This change has the problem that the magnitude of the reduction of momentum of the bit that is necessary after the last impact by the beat piece in order to bring the bit to rest in the tool holder is increased, with the result that the impact forces exerted on the retaining element by the rear end of the retaining groove of the bit are also increased, which can damage the components of the tool holder. The rear end configuration of the retaining groove of the bit is not, in fact "square" (i.e. normal to the bit shank axis) but instead is curved to form the arc of a circle in longitudinal cross-section. Furthermore, the retaining groove is relatively shallow, having a depth that is less than the radius of any spherical retaining element. This has the result that, when the rear end of the retaining groove of the bit hits the retaining element of the tool holder, the retaining element is forced both axially forwardly and radially outwardly.
The problem of damage to the tool holder caused by the impact of the retaining groove of the bit shank on the retaining element depends to a large extent on the distance that is available for the bit to decelerate to rest after having been hit by the beat piece. Thus, one solution would be to extend the length of the bit and the tool holder so that the rate of deceleration of the bit, and hence the force acting on the retaining element is reduced. However, such a solution is not possible without removing compatibility of the tool holder with existing designs of bits such as the SDS and SDS Plus bits which set the length of the bit shank and the retaining grooves used therein. The problem of the magnitude of the forces exerted on the retaining element by the rear end of the retaining groove of the bit shank is exacerbated by the fact that the retaining groove of the bit shank is relatively shallow, so that a force is generated in the radial direction that can be significantly larger than the force exerted by the rear end of the retaining groove in the axial direction.
The Bosch GSH 10 and Hilti TE 905 tool holder designs employ retaining elements that are located rearwardly of, and radially within, a restraining element which hits some means for absorbing movement of the restraining element when it is struck by the retaining elements. The means for absorbing the movement of the restraining element acts to absorb the shock of the impact of the restraining element, and normally comprises an insert formed from a deformable plastics material. Such designs, however, are designed for longer bit shanks such as hexagonal shanks, and for larger hammers, and cannot be adapted for shorter bit shanks without the problem of damage due to the rapid deceleration of the bit necessitated by the relatively short shank length.